The unraveling mystery of whether Donald Trump’s presidential campaign colluded with Russia just produced a smoking gun: those emails from Donald Trump Jr. welcoming an offer from Moscow to supply dirt on Hillary Clinton.

This wasn’t a casual meeting between the candidate’s impetuous son and some random peddler of political gossip. Trump Jr. was explicitly offered “sensitive information (as) part of Russia and its government’s support for Mr. Trump.” He recruited two other top aides, campaign Chairman Paul Manafort and Trump’s son-in-law Jared Kushner, to come along. All three were busy men; their presence suggests they considered the meeting to be a matter of high importance.

So now we know that the Trump campaign, at its highest level, eagerly sought Russia’s help.

There’s still plenty we don’t know, of course.

We don’t know whether that initial meeting in June 2016 led to other secret contacts. We don’t know whether the Trump aides’ willingness to hear an offer of clandestine help led to genuine collaboration.

In other words, we have a smoking gun -- but no bullet and no body. (Clinton’s defeat doesn’t count as evidence. She lost that election a half-dozen ways; Russian hacking was the least of them.)

And we don’t know the answer to the old Watergate question: What did the president know, and when did he know it?

But there’s lots of circumstantial evidence to suggest that acts of collusion may have occurred. The Russians hacked Democratic emails and released them through WikiLeaks, according to US intelligence. Candidate Trump approved the hacking publicly and urged the Russians to do more. A longtime Trump ally, Roger Stone, seemed to know in advance when the email releases were going to happen. And the releases were often cleverly timed -- just before the Democratic National Convention, for example. “It’s as if the Russians were being advised by somebody who knows how a presidential campaign works,” a veteran Democratic strategist told me. “Someone like Paul Manafort.”

Trump’s defenders will labor to put a charitable construction on what the emails revealed. It was only one meeting, they’ll say. If nothing resulted, it was unseemly and improper, but little more. Besides, Trump Jr. says the Russian lawyer who attended the meeting didn’t turn over any dirt. That means the meeting was an attempt at collusion that didn’t pan out. It’s also not clear that the lawyer was really acting on behalf of the Russian government. (But Trump Jr. thought she was, and the Kremlin often uses private citizens as cutouts, to preserve deniability.)

Finally, the candidate’s son and son-in-law, political neophytes, may not have known that they were potentially violating a federal law against seeking campaign help from foreigners. (Manafort, a veteran of many campaigns, should have known.)

What none of those excuses undercut, however, is that Donald Trump Jr. set out to obtain “sensitive information” from someone he thought was working for the Russian government.

“If it’s what you say,” he wrote, “I love it especially later in the summer.”

To a prosecutor, that’s evidence of intent, one of the elements necessary to make a case for criminal conspiracy.

“Conspiracy is a broad crime,” Jeffrey H. Smith, a former general counsel at the CIA, told me Tuesday. “There is no need that the crime actually occur, only that the individuals were intending to do it and took steps to carry it out. These emails come pretty close.”

Last week, Trump Jr. hired a criminal lawyer. That seems wise.

Moreover, the emails explode, yet again, the president’s long string of denials that anyone in his campaign was ever in contact with Russia.

For months, Trump has denounced allegations of collusion as “a made-up story” and dismissed the FBI investigation of his campaign as a “witch hunt” — even as his attorney general, his former national security adviser, his son-in-law and son have admitted to contacts with Russians that they once concealed.

Trump Jr. continued his family’s practice of clumsy, quickly evaporating denials, first claiming that he met with the Russian lawyer to discuss adoption policy, then admitting they discussed the campaign.

It’s simply baffling why any of the president’s luckless spokespersons, let alone any other self-respecting Republican, would continue to stake their honor on the Trump family’s honesty.

“There is no evidence of collusion,” Trump declared in May.

Now there is, at the very least, evidence of attempted collusion. The president can’t claim that the investigation is a witch hunt anymore. He’s in new, more dangerous territory. The Trumps’ own clumsiness has made it ever more likely that if they did something wrong, special counsel Robert S. Mueller III will find it.